Electrically fired gas expansion blasting device



March 1955 F. A. LOVING, JR

ELECTRICALLY FIRED GAS EXPANSION BLASTING DEVICE Filed March 12, 1953INVENTOR G, JR.

FRANK A. LOVIN ATTORN E Y United States Patent ELECTRICALLY FIRED GASEXPANSION BLASTING DEVICE Frank A. Loving, Jr., Wenonah, N. J., assignorto E. I. du Pont de Nemours and Company, Wilmington, Del., a corporationof Delaware Application March 12, 1953, Serial No. 341,985

1 Claim. (Cl. 102-25) This invention relates to an electrically firedgas expansion blasting device. More specifically it is concerned with animproved electrode for such a device.

Electrically fired gas expansion blasting devices are known to thetrade. A typical structure is described in U. 8. Patent 2,2U/,l9l. itconsists essentially of a head section, including insulated electrodes,a pressure buildup section adapted to accept a replaceable gasgenerating charge, and a gas discharge venting section. in theconventional structure, the head and the discharge venting sections areat opposite ends of the pressure build-up section. The pressure build-upsection and discharge venting section are separated by valving orpreferably by a frangible rupture disc. When readied for firing, theapparatus is positioned with its discharge venting section within a borehole of suitable diameter. The gas generating composition is then firedby means of a heating element to which current is supplied through theelectrodes. Usually the metal shell of the device constitutes one suchelectrode. The other is cored to the heating element through insulators,generally of the fiber washer type. The severe operating conditions, i.e., high temperature, violent mechanical shock, intimate contact withcorrosive gases, moisture generation and condensation and the like, havebeen observed to promote current leakage in the electrical circuit. Suchleakage results in hang fires and is therefore a hazard under operatingconditions.

In accordance with the present invention, it has been found that suchelectrical leakage can be greatly reduced by applying to at least thepressure finger of the cored electrode a ribbon coating of glazed,non-porous, vitreous enamel extending along the pressure finger adistance, in each direction from its point of protrusion into thepressure section, of at least about five times the thickness of thefiber insulation of the cored electrode.

The invention will be more fully understood by reference to thedrawings.

Figure 1 is an elevation partly in cross-section of a typicalelectrically fired gas expansion device;

Figure 2 is a sectional elevation of an electrode suitable for use inthe device of Figure 1, bearing a continuous coating of glazed,non-porous, vitreous enamel along its pressure finger, its firing headfinger and around its retaining ring;

Figure 3 is a sectional elevation of an electrode containing a ribbon ofglazed, non-porous, vitreous enamel along its pressure finger;

Figure 4 is a sectional elevation of an electrode having a continuouscoating of glazed, non-porous, vitreous enamel along its pressure fingerand also along the pressure face, side and the sealing face of itsretaining ring.

Referring to Figure l, the device is fired by igniting the gasgenerating charge, 1. This is accomplished by supplying current tofilament 2, through cored electrode 3 and metal electrode housing 4. Thecored electrode is insulated from its metal housing by fiber washers 5and air space 6. The electrode and washers are retained within the coredelectrode housing by retaining nut 7. The cored electrode housing actsas a connecting sleeve for the pressure build-up section wall 8 and thefiring head 9. Lead wire 10, containing insulated ice cores 11 connectthe device to an external source of current. The container 12 for thegas generating charge and its filament is normally loaded through thedischarge end of the pressure build-up section after removal of the gasdischarge venting section 13. The gas generating charge container isheld in place by metal retaining clips 14 which act to make electricalconnection of one end of the filament to the electrode housing. Theretaining clips are attached to the insulating container plug 15, as issocket 16 which makes electrical connection between the other end of thefilament and the cored electrode. Alternatively the retaining clip maybe integral with and of the same material as plug 15. Employing suchconstruction, the electrical. connection with the filament is made bypinching its projecting end between the plug and the electrode housing.

After the gas generating container is loaded into the device, thedischarge end of the pressure build-up section is sealed with afrangible disc (not shown). This disc is held in place by the gasdischarge venting section which is normally threaded to the pressurebuild-up section. Upon firing, a pressure build-up occurs until therupture pressure of the frangible disc is reached. Thereafter the gasexpands into the gas discharge venting section and escapes through ports17.

Figures 2, 3 and 4 are enlarged sectional elevations of the coredelectrode 3 each bearing a vitreous enamel coating 18. Figure 2represents the preferred embodiment wherein the enamel coating isapplied along the pressure finger 19, the firing head finger 20 andretaining ring 21. Suitable insulation free contact areas 22 areprovided at each end of the device for electrical contact purposes. Itis not essential that the coating be as extensive as that in Figure 2. Aribbon band along the pressure finger acts to reduce current leakage. Itis preferable to have it extend a distance, in each direction from thepoint of protrusion into the pressure section, of at least about fivetimes the thickness of the fiber washer insulator. The structure ofFigure 4 offers protection over both the pressure finger and theretaining flange. Since the discharge of the device tends to seal thefiring head section from the pressure build-up sec tion at the retainingflange, little or no enamel is necessary along the firing head finger.

By the term vitreous enamel as used herein is meant a fused silicatecoating. It may be applied by conventional methods to the coredelectrode by either the wet or dry process. The enamel should have ahigh glazing temperature. Enamels with glazing temperatures above about1400 F. are particularly effective. A glazing temperature Within therange of about 1400 to about 1600 F. is preferred. The normal glazingperiod of from about 1 to about 15 minutes in a dust free at mosphereunder the usual uniform heating and cooling conditions provides asatisfactory coating.

The thickness of the coating should be about at least /2 mil. It shouldnot exceed about 4 mils. A coating which is both non-porous andsufiiciently flexible to be shock resistant is of a thickness betweenthe limits of about 2 to 3 mils. Such a coating may be applied in asingle or multiple firing.

Various conventional components and additives in the nature ofrefractories, fluxes, opacifiers, colors, floating agents and peptizersmay be included within the coating composition. The cored electrodeproper may be of any metal. Preferably it is of steel because of thecombined properties of strength, conductivity and the strength of itsbond to enamel. Copper, cast iron and various other metals havingsuitable electrical conductivity may also be used.

The material employed as the gas generating charge may vary widely. Itmay be any liquid or solid substance or any combination of substanceswhich, upon the application of heat, will generate gaseous products.Liquid or solid carbon dioxide, liquid oxygen, ammonium nitrate and thelike are typical of such materials. However the electrode of the presentinvention is particularly valuable where one product of the gasgeneration is water or water vapor.

Many modifications will be apparent to those skilled in the art as aresult of the foregoing disclosure Without a departure from theinventive concept.

What is claimed is:

In an improved gas-expansion blasting device including anelectrode-housing at one end, an elongated terminal electrode having acentral portion of enlarged diameter with respect to its ends mounted insaid housing, and a washer electrically insulating said electrode fromsaid housing, said electrode being uniformly coated over its entiresurface except for a short contact section at each of said ends with a/2 to 3 mils thickness of a non-porous, glazed, vitreous enamel whichhas a glazing temperature of'at least 1400 F.

References Cited in the file of this patent UNITED STATES PATENTS1,736,398 Glossl Nov. 19, 1929 1,818,995 -Kneeland Aug. 18, 19312,163,119 Freeland June 20, 1939 2,207,191 Geertz July 9, 1940 2,213,969Ruben Sept. 10, 1940 2,215,295 Morrill Sept. 17, 1940

